An MRI apparatus is an apparatus that measures an NMR signal (echo signal) generated by an object, especially, the nuclear spins that form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In imaging, an object is disposed in a static magnetic field (polarized magnetic field BO), and then a slice selection gradient magnetic field pulse is applied together with a high frequency magnetic field pulse (RF pulse) in order to selectively excite a specific region and a phase encoding gradient magnetic field pulse or a readout gradient magnetic field pulse is applied for encoding within the excitation range, thereby giving the phase information.
The slice gradient magnetic field pulse generates a magnetic field inclined in an arbitrary direction, and generates a strength gradient of the magnetic field in static magnetic field space. The nuclear spins that form an object precess at a frequency corresponding to the gyromagnetic ratio and the strength of the inclined magnetic field. The frequency of the precession is called a Larmor frequency, and it is possible to excite only the nuclear spins at an arbitrary position by irradiating the RF pulse that matches the Larmor frequency.
In the case of an ideal state neglecting the relaxation phenomenon of nuclear spins, the excitation range and strength (hereinafter, referred to as an excitation profile) are a Fourier transform of the envelope of the RF pulse. The most common waveform of the envelope of the RF pulse is a Sinc function. Using a Sinc function for the envelope of the RF pulse, it is possible to obtain the rectangular excitation profile except for the truncation error.
Imaging is performed according to the pulse sequence set in advance. Among the pulse sequences, there is an ultra-short echo time sequence (Ultra-short TE Sequence; hereinafter, referred to as a UTE sequence) to measure the signal of tissue having a short transverse relaxation time (T2) (PTL 1, NPL 1, and the like). In the UTE sequence, a half RF pulse having a waveform corresponding to the half waveform of a normal RF pulse (full RF pulse) is used as the envelope of the RF pulse. In addition, this half RF pulse is irradiated together with a slice gradient magnetic field with an inverted polarity, two excitations are performed, and echo signals are measured twice according to the excitation and are added. Hereinafter, a slice gradient magnetic field applied with its polarity as a positive polarity is called a positive polarity slice gradient magnetic field, a slice gradient magnetic field applied with its polarity as a negative polarity is called a negative polarity slice gradient magnetic field, an echo signal acquired when the positive polarity slice gradient magnetic field is applied is called positive polarity data, and an echo signal acquired when the negative polarity slice gradient magnetic field is applied is called negative polarity data.
Side lobe in the excitation profile based on the half RF pulse is larger than that in the excitation profile based on the full RE pulse. However, since the excitation profiles of the positive polarity data and the negative polarity data have phase distributions that are 180 [deg] inverted with respect to each other in their side lobe portions, the side lobe signals are canceled out by adding the positive polarity data and the negative polarity data. Accordingly, it is possible to obtain the excitation profile equivalent to the excitation profile based on the full RE pulse.